PV module suffering from old age?

[salty]

Here's my kit; 2 Mitsubishi Electric 92W ,Voc 15.2V   I  7.52A ,10 years old , 2 Sharp 120W Voc 21V, 5 years old, all wired in parallel through a Bluesky 2000E MPPT charge controller to a 220 Ah battery bank. I have never had the performance I expected from this array and lately it got worse and when I investigated I found a loose connection and more. I discovered when I disconnected the Mitsubishi's my output went up and my MPPT function started working for the first time that I have seen. Stand alone the Mitsubishi's were putting out about 2A but when connected to the Sharps the total output dropped by a couple of amps. Both Mitsubishi's tested had Voc's of around 13.95V and the diodes seemed to be working  normally. Sharp panels alone are working great now, best performance I have ever had.
Questions; Are panels with different Voc ratings non compatible?
                  The Mitsubishi's alone never activated the MPPT function. Are they not compatible with the Bluesky 2000E?
                  Could the Mitsubishi's be suffering from old age after ten years?
                  If they are suffering from old age could they be rebuilt by putting new cells in?
                 
                 

[DamonHD]

I would have (Schottky) blocking diodes in series with each parallel string, *especially* those of different characteristics, at least in part to avoid the possibility of permanently damaging some of the them through being back-fed.

Note that such blocking diodes are different to bypass diodes: which do you believe that you have?

Rgds

Damon

[salty]

Damon, I'm not sure of how the panel is wired but I would think that they would have blocking diodes in them, which checked normal with my diode tester function on my multimeter.
                thx. salty

[thirteen]

what happens when you disconnect the sharp panels and does the performance increase with the Mitsubishi panels? Maybe a possible wire cross over or short?

[richhagen]

Many commercial panels I have bought have had bypass diodes wired in, this helps protect them if there is partial shading, especially in higher voltage strings, but I have always had to add a blocking diode to each string.  The manufacturers do not know what voltage your system will be and hence, how many panels you will be putting in series.  Since you would lose up to .5 volts or so per string for each diode in series, they do not generally add them to each panel.  A blocking diode would have to be wired so that all of the current flowing through the panel has to pass through it in order for it to function in that role.  Rich

[salty]

 Does a "string" imply a set of modules wired in series? My setup is all parallel wired. Can anyone say what the normal diode setup in a consumer panel would be. Could a panel function properly without a blocking diode in series?

[DamonHD]

A string can be as little as 1 panel: for the 12V nominal panels I use for my off-grid system, all in parallel, some come with series blocking diodes and some don't.  I ensure that all panels have a blocking diode once added to my system, ie if necessary I add one myself.  (Sometimes I don't trust the built-in one to fail open or not otherwise allow backfeeding, eg due to damp, and slightly wastefully add another Schottky: I should have been an actuary...)

Rgds

Damon

[salty]

So I have been checking out diodes in my parts collection today and got out my Fluke m/m manual and found that a long beep indicates a bad diode and short beep indicates ok. So the modules may indeed have faulty diodes as I got a continuous beep which would solve the power drop question. I have some IN 5401 diodes in my possession. Any idea if these would work as blocking diodes.

[oztules]

The mitsubishi's will barely charge  12V battery ... if at all. Wired all in parallel they will be useless.

You have an mppt, check the max input voltage. If if handles at least 15 + 21=36v... so say 40v, then wire the two mitsubishi in parallel, the two sharps in parallel, and then wire the two parallel units in series for nominal 30working volts@13working  amps into the mppt for about 400w output . You will loose an amp or so of the max out of the mitsubishi, but they will work properly for the first time. Two in parallel should give 15A, but the sharps will series limit this to about 13.2A


.............better than what you have now i suspect. The WVolts of the mitsubishi will be <13v.... you cant realistically charge with that direct to the battery.



................oztules

[salty]

Interesting solution,I like it. Won't work however as the Bluesky has a max Voc of 30v. Could I gain more from the Mitsubishi's by connecting them in series for a Voc of around 30v and paralleling them with the sharp PV's.

[oztules]

Yes thats workable, but you may be pushing the open circuit voltage of the mitubishi panels to over 30voc in cold weather. You could series some diodes with those panels to get the voc down a bit. typically .5v/diode can be dropped..... so 2 in series, will drop it 1v..... 18 in series will drop it to the same as the sharps 21voc..... sacrificing 60-70 watts all over the place..... a suck it and see test with the mppt to make sense of the inputs is probably in order. The panel interactions will be hard to guess accurately for the mppt. It may drive the mitsubishi panels down to meet the sharp, or it may never see the sharp..... and everywhere in between.

Remember, photon quantity (strength of light) dictates electron movement (current) and temperature affects the P-N junction electric field (voltage). Colder will drive the electric field voltage up across the P-N junction open circuit, warmer will drive the field voltage down.

I guess anything better than what you see now is a good thing.


..............oztules

[salty]

Sounds like a bit of a crap shoot. If the Mitsubishi's aren't compatible with the sharps and the mppt controller maybe they should have there own controller and all those electrons can finally meet at the batteries.

[birdhouse]

that would be the best route.  if you're buying another controller anyways, the morningstar mppt ts-45 has a Voc capability of 150v.  then you could put all your panels on it and have the blue sky as a spare, or get more panels for it?? 

adam

[salty]

Seeing as a Voc check on the Mitsubishi panels showed 13.9V in bright sun could I get away with running them without a charge controller and just have blocking diodes to prevent the batteries from lighting them up. I would disconnect the panels when away or demand is low.

[salty]

Update; So I wired the 2 mitsubishi panels in parallel direct to the batteries with no diodes and a switch to disconnect when no sun. They put out 2A at around 14 Voc , a whopping 28 watts of the 184 watts they are rated for. Could they be worn out? dead cells? Can they be rebuilt,repaired? Seems like there is some good hardware there. Coffee table?

[birdhouse]

bummer! 

i wouldn't go the coffee table route just yet...  you could keep the same scheme you've got now, but just wire them in series.  that would def. get you some more amps, though still ~1/2 of what they're rated for.  the upside, is that with a fairly large mismatch between two series panels versus battery SOC, you would begin charging earlier in the morning, and stay charging later in the day!

adam

[Mary B]

What voltage was the battery at? If it was almost full they won't put out much.

[salty]

Adam, I'm a little reluctant to run in series because I would have to run them through my charge controller and the series voltage would be close to the controllers max Voc.
MaryAlana, I also have 2 sharps feeding into the batteries through a controller which when in sun have the battery near 14V. So will the battery voltage regulate the panel output?If that is the case then maybe they aren't basket cases.

[Mary B]

Yes as batteries get full charge current tapers off towards zero.